I'm not sure about forms, since you didn't mention how you set those up on your website, but I can help you with the case where the Rule is called after saving a new comment with a Fivestar field, say it's called Stars. Follow the guidelines underneath for your Rule:
Events: After saving a new comment.
- Check whether the comment has a field Stars with Entity has field.
- Check whether the field Stars is not empty with NOT Data value is empty.
- Run this custom code:
Adapt these steps to your needs. Be sure to compare the rating with a score out of 100, since Fivestar works internally with percentages.